1. Field of the Invention
The invention relates to oil field tools. More particularly, the invention relates to downhole tools providing variable choking capability.
2. Prior Art
Oil wells can be productive to the point of over productiveness when the flow is not controlled downhole. Oil and gas in underground/under sea reservoirs are at extremely high pressure and can be all too willing to be expressed from these reservoirs. As one of skill in the art is painfully aware, this condition is hazardous and must avoided.
In order to prevent the outflow of oil or gas at a rate greater than can be accommodated at the surface and to control production of unwanted fluids, many systems have traditionally been employed. One of the tools that is used both to control the rate of expulsion of hydrocarbons from the reservoir and in some cases to limit the penetration into the well of undesired fluids is a choke. Chokes conventionally employ inner and outer sleeves having alignable and misalignable ports that are of the same size and shape. In these systems the degree of alignment of ports regulates the speed of the flow, thus how choked the system is. A drawback of such system is that erosion characteristics tend to make the system cost prohibitive.
A variable choke as disclosed herein employs, in the broadest sense, a choke housing and choke insert which are variably positionable relative to one another to align and misalign, to varying degrees, sets of ports in the housing and insert. Specially shaped and oriented ports provide for pressure equalization and choking capabilities while minimizing erosion of the components of the choke. In particular, a preferred port shape comprises a port and a subport depending therefrom. The subport is of smaller area than the port and preferably is elongated. An elongated subport reduces erosion of the subport itself when subject to flowing fluid because of fluid dynamics which cause the stream to become thinner than the actual dimension of the subport. Thus while fluid passes through the subport at high velocity the shape of the subport and its construction from an erosion resistant material, help to minimize erosion.
A further feature of the choke is that a seal stack is not subject directly to flowing fluid thus providing a longer life.
Finally, with respect to pressure equalization, the choke is resistant to the deleterious effects of equalization of a large pressure differential by incorporating at least one and preferably two diffuser rings to restrict flow and introduce turbulence which reduces flow velocity. These cooperate to allow the choke to effectively equalize a pressure differential.